Union "The Boolean operation failed." error

I am trying to union these two bodies together (the grey with the black outline near the bottom with the rest of the part) but getting an error that says, “The Boolean operation failed.”
What am I doing wrong and how do I fix it?

You can only union bodies that touch / intersect eachother.

Yes, they do touch and interact. I used the pen tool to outline a hole and extruded to fill it. It worked for all but 2 holes.

In this case, please open a support ticket and attach the file, we can investigate the problem.